1. Field of the Invention
The present invention relates to a continuously variable belt transmission device which is suitable for use for coupling the power output member of an engine of a vehicle such as an automobile to the driven wheels thereof, and more particularly relates to such a continuously variable belt transmission device in which an endless transmission belt is stretched around a driving pulley assembly and a driven pulley assembly and in which continuous variation of the transmission ratio is obtained by varying the groove widths of the driving pulley assembly and the driven pulley assembly in synchronization so as to vary the effective diameters of said pulley assemblies.
2. Discussion of the Background
There is a known form of continuously variable belt transmission device, which is generally structured as follows. A driving power input shaft and a driven power output shaft extend parallel to one another, with a driving pulley assembly provided on the driving shaft and a driven pulley assembly provided on the driven shaft. Each of these pulley assemblies is made up of an axially fixed pulley member and an axially movable pulley member, these two pulley members having conical faces which axially confront one another and being spaced apart by a distance which can be varied. Thus, in each of the driving and driven pulley assemblies, a V-shaped groove is defined between the two pulley members thereof, and the width of this V-shaped groove can be selectively altered. An endless belt is fitted around the two pulley assemblies, resting in their V-shaped grooves, and typically this endless belt is made up from a number of wedge shaped blocks each of which fits snugly into the V-shaped grooves, all linked together in series in an chain gang manner by one or more flexible and strong backing belts. With respect to the direction along the axes of the driving and driven shafts, in the driving pulley assembly the axial order of the axially fixed pulley member and the axially movable pulley member is opposite that in the driven pulley assembly. Such a continuously variable belt transmission device is exemplified in the Transmatic transmission of Van Doorn Ness Transmission Company of Holland.
In such a continuously variable belt transmission device, when it is desired to vary the transmission ratio, the widths of the V-shaped grooves of the driving pulley assembly and of the driven pulley assembly are simultaneously altered by a control system, so one of them is narrowed while the other is widened, so that the endless belt rides upwards away from the rotational axis of the one of the pulley assemblies while riding downwards towards the rotational axis of the other of the pulley assemblies, while always keeping the endless belt properly taut. In more detail, considering the unity one-to-one transmission ratio position, in which the endless belt is taut and the radius of its run which is extended around the driving pulley assembly is the same as the radius of its run which is extended around the driven pulley assembly, as the standard unity transmission ratio position (this term will henceforward be used for this state of the transmission), when it is desired to lower the transmission ratio from unity, in other words to provide a speed increasing power transmission function, the movable pulley member of the driving pulley assembly is brought closer to the fixed pulley member of said driving pulley assembly, so that the radius of the run of the endless belt which is extended around said driving pulley assembly increases, while at the same time the movable pulley member of the driven pulley assembly is moved further away from the fixed pulley member of said driven pulley assembly, so that the radius of the run of the endless belt which is extended around said driven pulley assembly decreases. This is done while keeping the endless belt properly taut around the pulley assemblies, by proper synchronization. On the other hand, when it is desired to raise the transmission ratio from unity, in other words to provide a speed reducing power transmission function, the movable pulley member of the driving pulley assembly is brought further away from the fixed pulley member of said driving pulley assembly, so that the radius of the run of the endless belt which is extended around said driving pulley assembly decreases, while at the same time the movable pulley member of the driven pulley assembly is brought closer to the fixed pulley member of said driven pulley assembly, so that the radius of the run of the endless belt which is extended around said driven pulley assembly increases. Again, by proper synchronization, this is done while keeping the endless belt properly taut around the pulley assemblies.
This type of continuously variable belt transmission device is effective for providing good and smooth power transmission with the transmission ratio being continuously variable without interrupting the transmission of power, in a smooth manner. However, it is a fact that, as the V-shaped groove of either of the driving or driven pulley assemblies is altered in width, not only does the radius of the run of the endless belt which is extended around said pulley assembly alter as the belt rides up or down between the pulley members thereof away from or towards the central axis thereof, but also, undesirably but inevitably, the center of the V-shaped groove of the pulley assembly is axially shifted by a distance equal to half the travel of the movable pulley member thereof; and this causes the run of the endless belt around said pulley assembly to be axially shifted in accordance therewith by the same amount. This is the reason that, as specified above, with respect to the direction along the axes of the driving and driven shafts, in the driving pulley assembly the axial order of the axially fixed pulley member and the axially movable pulley member is arranged to be opposite to that in the driven pulley assembly. This particular arrangement means that as the transmission ratio is either increased or decreased from unity the center of the V-shaped groove of the driving pulley assembly is shifted in the same direction as the center of the V-shaped groove of the driven pulley assembly along the axes of the respective shafts, and therefore to a first approximation these motions cancel one another and do not axially twist the endless belt.
In accordance with this, it has been conventional to arrange the length of the endless belt and to configure the continuously variable belt transmission device as a whole so that, when the V-shaped grooves of the driving and driven pulley assemblies are both set to the same width and accordingly the radiuses of the runs of the endless belt which are extended around said pulley assemblies are substantially the same and therefore a power transmission ratio of unity is being provided (the standard unity transmission ratio position), the centers of said V-shaped grooves are coincident in the axial direction of the driving and driven shafts, so that the planes of symmetry of the driving and driven pulley assemblies are coincident. Thus, in this standard unity transmission ratio position, no skewing is caused to the endless belt, and good and smooth transmission is provided. And, when the transmission ratio is altered from this unity position, either upwards or downwards, by widening one of the V-shaped grooves of one of the pulley assemblies and simultaneously narrowing the other, as explained above to a first approximation the centers of the V-shaped grooves are moved to the same amount in the same direction, thus not causing any skewing.
However, the problem with this construction is that the magnitude of the movement of the center of the V-shaped grooves is in fact only the same to a first approximation. Considering a small alteration in transmission ratio from a position of the continuously variable belt transmission device which is not the unity transmission ratio position so that the radiuses of the runs of the endless belt which are extended around the two pulley assemblies are not equal, for example in the direction to further increase the difference of the transmission ratio from unity, then it will be understood that (in order to maintain the endless belt properly taut) the amount of tautening movement towards one another required between the movable and the fixed pulley members of that pulley assembly which at this time is providing the greater pulley radius, is less than the corresponding amount of loosening movement away from one another required between the movable and the fixed pulley members of that pulley assembly which is providing the lesser pulley radius. This is because the run of the endless belt around that pulley assembly which is providing the greater pulley radius extends through a greater angle around its center than does the run of the endless belt around that pulley assembly which is providing the lesser pulley radius, and is an inevitable consequence of the construction, assuming that the distance between the shafts of the two pulley assemblies is kept constant, that the length of the endless belt is fixed, and that the width of the V-shaped groove of each of the pulley assemblies is altered by the movement of only one of its pulley members on its one side.
Therefore, since during alteration of transmission ratio the centers of the V-shaped grooves move through different corresponding distances, it is inevitable that skewing of the endless belt will occur at some times; but however is highly undesirable for the reason that load transmission during skewing causes severe strains to be put on the endless belt. In particular, in the case of an endless belt assembly structured as suggested above, when power is being transmitted with the endless belt assembly skewed, i.e. with the centers of the V-shaped grooves of the pulley assemblies being offset from one another in the direction of the axes of the shafts, then the backing belt or belts of the endless belt assembly, which typically is flexible in one direction but very stiff in a transverse direction, may be severely strained. Further, the sides of the wedge shaped blocks of the endless belt assembly are caused to scrape against the sides of the pulley members in a misaligned fashion. Particularly, if as is conventional the wedge shaped blocks are formed with slots cut into their sides between which slots there remain neck portions, and if two of the backing belts are provided, one fitted into the ones of these slots in each of the longitudinal directions of the shafts, then operation of the endless belt assembly under load in a skewed position may result in one of the backing belts being twisted and skewed so much as to come into contact with either the neck portions of one or more of the blocks, or with the conical face of one of the pulley members of one of the pulley assemblies. In either case, severe damage to the transmission can result. And, since both the maximum speed of power transmission through the continuously variable belt transmission device, and the maximum attainable torque, are also critically dependent upon non skewing of the belt assembly thereof, the conventional construction is also limited in respect to these respects.